In seismic surveying, geophysicists direct mechanically generated, low frequency acoustic energy waves into the earth and record the waves reflected back to study the structure and nature of the rock layers below the surface. In typical 3D seismic surveying at sea, a seismic vessel tows a series of seismic cables or streamer cables behind it along with at least one energy source. Further, acoustic signals are generated by a series of air guns, arranged in a subarray, which is also towed behind the seismic vessel. Data about the geological formations at or below the ocean floor is gathered by sending sound waves from one or more sources down into the sea bottom. The various types of formations reflect the waves back differently. The sensors on the streamer cables of the marine seismic array receive the reflected signals for subsequent analysis. The seismic vessel tows the seismic array of streamer cables along a specified path to perform the desired survey in a chosen area. As such, an efficient towing configuration of the marine seismic array is desirable, e.g., minimal drag, where the streamer cables remain at their assigned depth and lateral offset.
At present, a typical configuration of marine seismic arrays is an eight-streamer configuration. In the prior art, the mechanisms used to maintain the cable separation include paravanes and smaller diverters. Traditionally, the towing method consists of two main paravanes acting as port and starboard side diverters, with the paravanes deployed on dedicated synthetic tow ropes. The two main paravanes are used to provide the lift force necessary to separate multiple streamers across a wide lateral swath. In this classic cable configuration, a chain of taglines or separation ropes is used to daisy-chain one streamer to the next. This chain of taglines starts at the paravane and leads inboard.
Another known towing mechanism is to replace the main paravanes with smaller diverters. This method is similar to the paravane configuration in that it can duplicate the same number, length, and separation of streamers. These smaller diverters are towed from the lead-in cables themselves instead of individual main paravanes on the port and starboard sides that are deployed from dedicated tow ropes. This configuration eliminates the drag of the tow ropes, and in some cases, also eliminates the drag from some of the separation ropes as well. Although the cumulative effect of this configuration is an incremental reduction in towing burden, this configuration still does not address the current and future demands of seismic array towing.
To improve efficiency of seismic surveying at sea, the spreads of streamer cables continue to grow both in terms of the number of streamers deployed and the separation between adjacent streamers. At the time of writing, typical configurations for large 3D surveys are 10 to 12 streamer cables, with expectations that this will increase in the future to 14 or 16 streamer cables for the most modern vessels. As such, either adding streamer cables or spreading streamer cables wider increases the towing burden of the seismic vessel and requires more lift from the paravanes or other similar means. With a conventional paravane, more lift translates to a larger device and larger, more sophisticated handling and rigging techniques. Larger paravanes mean more room is taken up on the seismic vessel to store the paravanes, and more fuel is consumed to tow them and the rigging equipment.
Further, deployment of seismic arrays with more streamer cables and/or more separated streamers will require substantially more time in stacking operations to recover the paravanes or various cables for inspection, maintenance, or repair. Recovery of the main paravanes is typically conducted every 6 weeks, which causes a delay in survey operations for 12 to 24 hours. The stacking operation of the seismic array using conventional paravanes for lift is additionally time consuming because it requires the following steps: (1) further release, or pay-out, of the lead-in and streamer cables on one side, e.g., port side, for extra spread to accommodate the cables to be stacked, e.g., starboard side cables; (2) hauling in of the lead-in cables to be stacked to exchange long separation ropes between the cables for shorter taglines; and (3) repeating steps (1) and (2) in reverse to re-deploy the recovered paravane or streamer cable once it is inspected and/or repaired.
The novel design of the present invention overcomes these problems associated with mechanisms known in the art and can be applied to a marine seismic array of as few as two streamer cables. For example, conventional paravanes produce substantial drag because they require tow ropes, vane tethers, and separation ropes. The present invention reduces the drag significantly by eliminating the conventional paravanes themselves, along with the gear required to operate them with the rest of the array. As such, a towing configuration using the present invention can be expected to generate up to 35% less drag than the conventional paravane configuration. This drag reduction will result in reduced fuel consumption for the seismic vessel, faster achievable production speeds for the vessel, and increased flexibility to tow more aggressive arrays, e.g., longer or more streamers and wider area of separation between the streamers. Further, due to the reduced drag and improved efficiency of the present invention, the cumulative wing area of the multiple deployed wing units of the present invention can be reduced by up to 50% as compared to conventional paravanes. As such, there is a reduction in the required stowage space on the seismic vessel.
Moreover, by eliminating the use of paravanes, it is no longer necessary to perform stacking operations every 6 weeks to inspect the paravanes, thereby eliminating the periodic interruptions in data collection that occur in prior art. While stacking is still required from time to time to repair or replace various cables, the present invention provides for an improved stacking operation that significantly reduces the necessary time to complete the operation. By eliminating the conventional paravanes themselves, along with the necessary operating gears such as separation ropes, additional pay-outs of cables are no longer necessary during the stacking procedure. The present invention also eliminates the required replacement of longer separation ropes with shorter tag lines. Further, by employing at least one steerable wing unit on each lead-in cable that can perform a 180 degree roll, the lead-in cables of the present invention can be forced to move in the opposite direction, e.g., from a port side lateral position to a starboard side lateral position, or vice versa. One skilled in the art will realize that other design techniques can be employed to achieve the same purpose of being able to steer a streamer across the center line of the towed equipment spread. As such, stacking of the cables can be performed quickly and easily without the need to retrieve the cables to be stacked.
Further, in conventional configurations, the lateral separations between cables are set at the time of deployment via separation ropes with fixed lengths that will likely stretch over time, leading to significant deviations in the pre-established separations. The only solution is to recover the ropes, re-adjust their lengths, and re-deploy the array. This process can cause a delay in the surveying operation of 4 to 8 hours. The present invention eliminates the use of separation ropes, thereby saving the cost of rope replacements and the time required to replace these ropes.
Also, with at least one dedicated wing unit per lead-in cable, each successive streamer cable can be deployed to its production position and left in place. There is no longer a need to partially recover the cables each time taglines and separation ropes are connected or disconnected, as in the conventional methods in the prior art. This significantly reduces deployment and retrieval times for the whole marine seismic array. Further, by eliminating all taglines, the present invention eliminates the risk of chafing of the array that can result in subsequent partial or total collapse of the seismic array. Also, the present invention provides for steerable wing units, providing the flexibility to control the attack angle of each wing unit and the lateral spacing of individual lead-in cables and streamer cables.
In view of the growing interest in super-wide arrays, the increased performance demands of conventional rigging technologies, and the limitations of known products, there is a great need for a durable, efficient, and cost-effective towing system to meet the challenging requirements of super-wide arrays. The present disclosure provides a system and method that reduces the towing burden and fuel consumption required for towing seismic array of any size. Further, the system and method disclosed herein overcomes the challenges of deploying and towing new super-wide arrays having more than eight streamers and/or larger separations. Additionally, the present disclosure provides a number of other advantages and benefits over the current configuration employing smaller diverters especially in terms of overall production efficiency.